In rotary offset printing, a web offset press applies ink to an image plate mounted on a plate cylinder. The image plate transfers ink to a resilient blanket on a blanket cylinder. The blanket imprints a paper web with the ink. The plate and blanket cylinders have to hold the image plate or blanket on the associated cylinder surface. Cylinders have been used which hold the plate magnetically. Magnetic cylinders must have sufficient holding capability for reliable operation in rotary web offset printing.
As discussed in Peekna et at., U.S. Pat. No. 4,676,161, owned by the assignee hereof, a typical blanket for a magnetic cylinder includes a carrier plate of ferromagnetic material. A blanket sheet is bonded to an outer surface of the carrier plate. A magnetic cylinder comprises a cylindrical core with peripheral axially spaced permanent magnets. Adjacent magnets have opposite polarity. Pole pieces of magnetic material are provided between adjacent magnets. The permanent magnets, pole pieces and the plate form magnetic circuits in which the flux established by the permanent magnets substantially saturate the peripheral faces of the pole pieces and annular sections of the plate between adjacent pole pieces.
As described in the Peekna et al. patent, the magnetic circuits were optimized to resist peeling the image plate or blanket carder plate off the cylinder. Additionally, the magnetic circuits were designed to suppress circumferential blanket movement.
Particularly, slow circumferential movement of a blanket on a magnetic cylinder has been observed under some printing conditions. Including also the results of laboratory investigations, several features of the slow circumferential movement have emerged, as follows. With a bare steel image plate on a magnetic cylinder rolling against a blanket on the other cylinder, movement is in the same direction as a tangential force on the plate in the nip. The tendency of a bare steel plate to move on a magnetic cylinder is enhanced by decreasing the normal nip load. With a blanket laminated to a steel carrier plate on a magnetic cylinder, movement is always opposite the direction of rotation irrespective of the direction of the tangential force in the nip. The tendency of a blanket laminated to a steel plate to move on a magnetic cylinder is enhanced by increasing the normal nip load. Introducing a layer of oil between a blanket carrier plate and a magnetic cylinder had a suppressing effect on movement. The behavior of a bare plate is accounted for by a sliding phenomenon. More particularly, by a sliding wave, in which sliding takes place only over a small area at or adjacent to the nip, with the plate acquiring a residual compressive strain to one side of the sliding wave, and residual tensile strain on the other side. The behavior of a blanket is hypothesized to be related to a lifting wave phenomenon in a small area adjacent to the nip. The carrier plate length along the lifting wave is slightly longer than the cylinder surface under the lifting wave. Accordingly, the blanket and carrier plate move along with the nip a small distance with each cylinder revolution. Movement with the nip along the cylinder is opposite the direction of rotation. Thus, a lifting wave always moves opposite the direction of rotation, no matter what the cause or provocation. Introducing a layer of oil or other partial seal impedes the flow of air into the space underneath an incipient lifting wave, thereby tending to inhibit its initiation.
The present invention is intended to overcome one or more of the problems discussed above.